1. Field of the Invention
The present invention relates to an automatic gain control (AGC) circuit and particularly to a video AGC circuit for use in a video unit such as a video tape recorder, a video disc player and the like.
2. Description of the Prior Art
In a video tape recorder, for example, the magnitude of a luminance signal (a composite video signal) applied from a video camera or a television tuner needs to be made equal to a certain value so that the signal may be frequency modulated and recorded. For this purpose, a video AGC circuit is generally utilized.
As such video AGC circuits, various types have been proposed. Recently, there is a growing demand for an AGC circuit of the type giving preference to keyed AGC operation in combination with peak AGC operation as indicated for example in pages 102 to 105 in "National Technical Report Vol. 25, No. 1" (published in Feb., 1979)
An AGC circuit of this type is generally constructed as shown in FIG. 1. Referring to FIG. 1, the reference numeral 20 denotes a video AGC circuit; the reference numeral 1 denotes an input terminal of a composite video signal; the reference numeral 2 denotes a gain control amplifying circuit; the reference numeral 3 denotes an output terminal of a gain controlled composite video signal; the reference numeral 4 denotes a synchronizing signal separation circuit for separating horizontal and vertical synchronizing signals from a composite video signal; the reference numeral 5 denotes a sync-tip clamp circuit for clamping to a certain potential the sync-tip level of a synchronizing signal of a video signal provided from the above stated control circuit 2; the reference numeral 6 denotes a delay circuit for delaying the above stated synchronizing signal by approximately 2 .mu.sec.; the reference numeral 7 denotes a pulse mixing circuit for mixing a reference pulse with the composite video signal from the clamp circuit 5; the reference numeral 8 denotes a detecting circuit for detecting the peak of the output signal of the pulse mixing circuit 7; the reference numeral 9 denotes a comparator for comparing a peak potential at the point B of the peak detecting circuit 8 and a reference potential at the point A preset by a variable resistor VR; and the reference numeral 10 denotes a connection line for controlling the above stated control circuit 2 according to the output of comparison.
In a conventional AGC circuit shown in FIG. 1, the level of a reference pulse applied thereto is always fixed to a certain value even if the reference voltage level for automatic gain control operation (AGC operation) is changed by a variable resistor VR. As a result, a disadvantage is involved that desired AGC operation giving preference to keyed AGC in combination with peak AGC might become impossible dependent on the state of a composite video signal.